1. Field of the Invention
The present invention relates to a cyclone apparatus for classifying and collecting a powder, and more particularly to a cyclone classifier and a flash drying system for drying and preparing a toner.
2. Discussion of the Background
Recently, powder is required to have sophisticated features such as a small particle diameter and a sharp particle diameter distribution. A powder having a broad particle diameter distribution has various uneven performances. The powder preferably has a uniform particle diameter to have high performances. A toner having a broad particle diameter distribution for use in electrophotography is also disadvantageous for its required uses such as being uniformly charged and melted.
Many classifying methods are known for making the particle diameter uniform. The classifying methods include a method of using a cyclone collector. Typically, the cyclone collector is used as a solid-gas separating apparatus. A powder transferred into a cyclone classifier using an airflow centrifugally accumulates on the wall of an outer cylinder with a swirling flow and gradually drops in a container installed at an under part of the outer cylinder of the cyclone classifier. The gas, which is much lighter than the particle (mostly air), is discharged out of the cyclone classifier from an inner cylinder in the center thereof.
A classifier using the cyclone collector for separating a solid from a gas, which discharges a powder having a small particle diameter together with the gas is also known. The cyclone collector is used for separating a solid from a gas and transporting a powder. A cyclone collector having an additional classifying function has an advantage of reducing capacity investment and man-hours.
The cyclone collector handles a powder having a particle diameter not greater than 1 mm.
Japanese Laid-Open Patent Publication No. 10-230223 discloses a classifying method of using a filter effect by placing a cylinder having pores between an outer cylinder and an inner cylinder of a cyclone collector. Japanese Laid-Open Patent Publication No. 8-2666938 discloses a method of controlling a classifying particle diameter by changing a gap due to pitch, wherein a slide plate changing the opening width of an entrance of a cyclone collector is arranged and the tip of a circular cone and is located facing the lower end of an outer cylinder of the cyclone collector. Further, Japanese Laid-Open Patent Publication No. 2004-283720 discloses a method of collecting an air stream including a powder in the center of the inner cylinder by increasing a flow speed with a division plate having an orifice having an area smaller than that of an end-opening of an inner cylinder, which is concentrically located in the center of an outer cylinder.
Controlling the classifying particle diameter is one of the important functions of a cyclone classifier, and a more important thing is how a powder is distributed in the order of particle diameter from smaller to larger toward the circumferential surface of anouter cylinder with a centrifugal force.
A powder having a larger particle diameter receives a stronger centrifugal force. Therefore, it is ideal that the powder having a smaller particle diameter is distributed in the center of the outer cylinder, i.e., around the inner cylinder of the cyclone classifier, and the powder having a larger particle diameter is distributed around the circumferential surface of the outer cylinder in the order of particle diameter almost continuously. When the classification point is controlled, a good-yield classifier and a classifying process separating powder having a sharp particle diameter distribution can be provided. In other words, it is necessary that a powder is specifically distributed in the order of particle diameter from the center to the circumferential surface of the outer cylinder, otherwise the powder cannot be classified even when the classification point is controlled.
In the method disclosed in Japanese Laid-Open Patent Publication No. 8-2666938, the opening width can be narrowed. However, when toner having different particle diameters is being mixed and gathered and already receiving centrifugal forces, the toner cannot be classified.
Even when a powder having a wide particle diameter distribution receives a centrifugal force on a swirling flow in the cyclone classifier when flown into the outer cylinder of a cyclone classifier, the powder cannot be classified to have desired particle diameters. This is because particles having various particle diameters, which come from the entrance varying in size, are nonuniformly mixed at a radial position where they begin to receive centrifugal forces. When a centrifugal force is further applied to the particles (the particles stay longer in the outer cylinder of the cyclone classifier), almost all the particles thinly gather on the inner wall of the outer cylinder and cannot be classified.
Because of these reasons, a need exists for a cyclone classifier capable of separating a powder having a sharp particle diameter distribution at a high yield.